1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for fabricating the device. As used herein, the “liquid crystal display devices” include both a direct viewing liquid crystal display device and a projection type liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (LCD) conducts a display operation by utilizing variations in the polarization states of an incoming light ray being transmitted through the liquid crystal layer thereof. The polarization state of the incoming light ray is changed by orientation directions of liquid crystal molecules, which in turn are changeable with the application of a voltage to the liquid crystal layer. In the LCD, a display panel, including the liquid crystal layer and electrodes and circuit components for use to apply a voltage to the liquid crystal layer, will be referred to herein as an “LCD panel”. The LCD includes not only the LCD panel but also a driver circuit, a power supply circuit and a light source. Portions of the driver circuit and power supply circuit may either form integral parts of the LCD panel or be mounted on the LCD panel.
The LCD panel typically includes two substrates and a liquid crystal layer provided between the substrates. Each of the two substrates includes an alignment film that faces the liquid crystal layer. The alignment films are provided so as to align liquid crystal molecules in a predetermined direction in the liquid crystal layer.
The alignment films are normally formed in the following manner.
Specifically, first, an organic polymer film of polyimide, for example, is deposited over a substrate on which electrodes for use to apply a voltage to the liquid crystal layer and circuit components (such as switching elements and lines) for supplying a predetermined voltage to the electrodes are provided. Next, the surface of this organic polymer film is mechanically rubbed directly with a cloth in a predetermined direction (subjected to a rubbing treatment), thereby obtaining an alignment film having the function of aligning the liquid crystal molecules in the predetermined direction.
It is believed that the alignment film should define the orientation direction of the liquid crystal molecules due to the shape effects of fine grooves that have been formed on the surface of the film as a result of the rubbing treatment, the stretching effects of organic polymers that make up the film, and the anisotropic electrostatic effects induced on the surface of the film.
However, if the liquid crystal molecules are aligned by the conventional rubbing treatment, then fibers of the cloth and impurities such as debris and dust may be deposited on the surface of the film or on the substrate, thus possibly causing some defects or deterioration on the display and decreasing the yield or reliability. Also, the static electricity, produced during the rubbing treatment, may cause a dielectric breakdown in thin-film transistors (TFTs) or metal-insulator-metal (MIM) elements, thus also bringing about some defects in the display. Furthermore, in the rubbing treatment, the pressure cannot always be applied sufficiently uniformly but may be applied locally non-uniformly. As a result, the liquid crystal molecules may have their pretilt angles disturbed to form some rubbing stripes in very small domains of the liquid crystal layer. In that case, the display quality may be seriously affected.
Furthermore, the rubbing treatment is preferably carried out in an environment that is as free from static electricity or dust as possible. Thus, in the actual production line, the rubbing treatment needs to be performed separately from the place where the process step of forming the organic polymer film is being carried out. In addition, after the rubbing treatment process has been carried out, the substrate needs to be subjected to a wet cleaning process step, which requires a huge quantity of cleaning liquid. Consequently, the conventional rubbing treatment significantly increases the number of required process steps and the cost of the LCD panel manufacturing process.
Thus, to overcome these problems, various non-contact alignment treatment techniques have been proposed. A method of forming an alignment film by exposing an organic polymer film, including photosensitive molecules, to a polarized ultraviolet ray is disclosed in Japanese Patent No. 2608661 and Japanese Laid-Open Publication No. 9-197406, for example.
Also, a so-called “optical alignment technique” of controlling the pretilt angle to be defined by a polyimide film for liquid crystal molecules by obliquely irradiating the polyimide film with a (non-polarized or polarized) ultraviolet ray is disclosed in Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A. 333, 165 (1999).
Furthermore, techniques of forming an alignment film by irradiating an alignment film with another energy beam such as an electron beam, an ion beam or a laser beam onto the surface of a target film unlike the optical alignment technique described above are disclosed in Japanese Laid-Open Publications No. 2-222927, No. 6-130391, No. 7-56172, and No. 9-218409, for example.
In any of these alternative techniques, when exposed to the energy beam, the surface of the alignment film is physically etched anisotropically, thereby forming a great number of fine grooves. Thus, the liquid crystal molecules are believed to be aligned anisotropically along those grooves. According to these energy beam methods, however, some damage is also done on the material of the film being exposed to the energy beam. Accordingly, alignment defects may be formed or the reliability may decrease. For that reason, it is difficult to optimize the energy beam irradiation conditions.
Japanese Laid-Open Publication No. 11-271773 discloses a method of arranging the atomic structures of an alignment film in a desired direction by exposing a film, which has been formed on a substrate by a dry patterning process (e.g., an, evaporation process, a sputtering process, an ion beam deposition process, a CVD process or a PECVD process), to a particle beam. In this method, the material of the alignment film must be optically transparent and amorphous or fine particular. Examples of specific alignment film materials disclosed therein include glass, graphite, diamond, SiC, SiO2, Si3N4, Al2O3, SnO2, InTiO2, InZnO2 and ZnTiO2. On the other hand, Japanese Laid-Open Publication No. 11-271774 discloses a technique of aligning liquid crystal molecules in which a film, producing bonds to be aligned on the surface preferentially, is formed and the surface thereof is exposed to (the impact of) a particle beam consisting of atoms, molecules, ions or clusters, thereby producing bonds that are aligned in a direction preferentially and anisotropically. Examples of specific alignment film materials disclosed therein include graphite, diamond, SiC, SiO2, Si3N4, Al2O3, SnO2, InTiO2, InZnO2 and ZnTiO2.
In the techniques disclosed in Japanese Laid-Open Publications No. 11-271773 and No. 11-271774, however, a film having a basically isotropic structure is exposed to a particle beam, thereby producing a preferential alignment for the atomic arrangements or bonds and aligning the liquid crystal molecules by utilizing the anisotropy. Thus, according to these techniques, the anchoring force is relatively weak and good reliability may not be achieved. Also, although Japanese Laid-Open Publication No. 11-271774 describes that a crystalline or polycrystalline material may contribute to forming the alignment film either indirectly or directly, the publication provides no specific disclosure about that.
Furthermore, Japanese Laid-Open Publications No. 2-294618 and No. 2001-21891 disclose a technique of adding an alignment film function to conductive films that are used to apply a voltage to a liquid crystal layer. Specifically, Japanese Laid-Open Publication No. 2-294618 discloses that transparent conductive films of ITO, which sandwich the liquid crystal layer, may have anchoring force when formed by an oblique evaporation process. On the other hand, Japanese Laid-Open Publication No. 2001-21891 discloses a method of providing anisotropy for conductive films by exposing the surface of electrodes made of an inorganic material such as ITO, Al or an Al alloy to an energy beam directly and thereby etching the surface of the electrodes anisotropically.
However, the conventional conductive film is normally an amorphous film or a polycrystalline film with random crystallographic orientations. Thus, it is difficult to maintain sufficient anchoring force, contributing to aligning the orientation directions of liquid crystal molecules constantly, with good reproducibility just by carrying out the oblique evaporation process as disclosed in Japanese Laid-Open Publication No. 2-294618 or by patterning the surface of the conductive films into an anisotropic shape (e.g., steps or grooves) by an anisotropic etching process as disclosed in Japanese Laid-Open Publication No. 2001-21891. In an active-matrix-addressed liquid crystal display device, in particular, the orientation directions of liquid crystal molecules are often disturbed near steps that are created by the complicated stacking structure. Accordingly, to make a commercially viable product, the anchoring force thereof must be increased.